Frequency transposer or converter and circuit therefor



Dec. 24, 1940. F. H. STIELTJES FREQUENCY TRANSPOSER OR CONVERTER AND CIRCUIT THEREFOR 2, 1937 3 SheetsS'neet 1 Filed Dec.

ATTORNEYS Dec. 24, 1940. F. H. STIELTJES 2,225,025

FREQUENCY TRANSPOSER OR CONVERTER AND CIRCUIT THEREFOR Filed Dec. 2, 1957 3 Sheets-Sheet 2 FREQUENCY FREQUZ-AICY 2 l N V EN TO R fkfaa /K f/f/mm/r' Jr/a was AJTO RNEYSD Dec. 24, 1940. F. H. STIELTJES 2,226,026

FREQUENCY TRANSPOSER OR CONVERTER AND CIRCUIT THEREFOR ATTO R N EY5.

Patented Dec. 24, 1940 UNITED STATES FREQUENCY TRANSPOSER OR CONVERTER AND CIRCUIT THEREFOR Frederik Hendrik Stieltjes, The Hague, Netherlands Application December 2, 1937, Serial No. 177,762 In the Netherlands December 4, 1936 10 Claims.

The invention refers to frequency transposers or converters of the type in which one or more input frequencies are output into one or more desired transposed frequencies by means of a transposing frequency. More in particular the invention refers to such transposers in which the frequencies generated caused by retroaction further undesired frequencies to be generated besides the original frequencies supplied for transposition.

An important group of such transposers are modulators; the subgroup known as inversion modulators for instance satisfies the above description. Primarily the present specification will for simplicitys sake be limited to this subgroup. The latter part of the specification will indicate to which further groups of transposers the inventive idea, as exemplified with reference to the example of the subgroup of inversion modulators, may be applied.

If in the example an inversion modulator is functioning with a transposing frequency in this will produce output frequencies fo+f1 and fo-fr and a number of undesired frequencies,

2.5 where the frequency fl is supplied at the input. If a single side band is desired one of the above specifically mentioned output frequencies might also be undesired. If the inversion is absolutely discontinuous the group of undesired frequencies will extend to infinite frequencies; for the present it will be assumed that the inversion is of this nature. Then a supplied sinusoidal voltage f1 will be converted into a voltage which is the product of the supplied voltage and the inversion factor, which is a function of the basic frequency in, and which varies from +1 during one half period to 1 during the next half period. I

If these voltages work at the output on im pedances having resistance components, energy losses will occur in the same. If on reactances which are not very high, the currents produced in the same will react on the input, there producing currents of frequency f1, which will in general not cancel out. These currents will be in quadrature with the voltage, and therefore will not cause energy dissipation. They will however flow through the internal, resistance of the source of the voltagef1,thereby causing secondary energy losses. In order to avoid these losses the said reactances should therefore be high; as the original voltage f1 has been supposed to be a pure sinusoid, and as the input currents of undesired frequencies will certainly occur, the reactances 5 for these frequencies will certainly have to be low at this side. It may be simply demonstrated at once that these undesired currents will occur at the input from the fact that in the simple case considered where high reactances for the undesired frequencies were assumed atthe out- 5 put for all undesired frequencies at that side, the output current will be a pure sinusoid, which is only possible if a mixture of currents of an infinite frequency range occurs at the input: in

general it will be seen that undesired currents l0 will be admitted at the input so soon as the output current does not exactly conform to the output voltage, which in its turn is equivalent to' the input voltage multiplied by the inversion factor. 15

The application of the invention as now described there-fore results in a modulator, which if placed between a source of a modulating frequency and a consumer of modulated frequency, will not cause primary or secondary energy 20 losses, if that source and the consumer are adapted to each other for their respective frequencies. As will appear below, this will be the case if the output impedance for the desired frequency is 4/7r as great as the input im- 25 pedance for the desired frequency; The diagram of Fig. 1 will in that case have to be considered as a reflectionless quadripole with frequency transformation, this quadripole conducting of course as well from left to right as from right 3 to left. By analogy it will be plausible that the location of the high and low reactance as described above may be interchanged.

It will be clear without further illustration that the invention may also be applied to in- 35 version modulators in which the inversion factor mentioned above is a difi'erent time function, e. g.

a staged function. This is true especially when the inversion is obtained by means of taps on a transformer or the like. 40

It further appears that the invention is likewise applicable to make and break modulators, as well as to such modulators working with complete interruption as to those where the periodic interruption is not absolute. In order to denote 45 on the other hand the limitations of its application, it is observed that a simple interruption modulator of the first kind will not be suitable for the application of the invention, for such a modulator would not function at all if provided 5 with reactances according to the invention. It is seen at once that such is the case if it be borne in mind that at both ends of such a modulator the current would periodically be fixed in value, viz. at zero, which evidently will only be possible if either the currents are identically zero, or beside the currents of desired frequency currents of other frequency may fiow. This requirement is not to be reconciled with what has been said in the above about the reactances to be connected.

This limitation is not present in those interruption modulators the input or output terminals of which are periodically shorted, viz. whenever the current path is interrupted. At those terminals the voltage will then be periodicaly zero, so that here voltages of other frequencies must also be allowed to occur, i. e. at this side high reactances will have to be connected for those frequencies. At the other end, where the current will periodically become zero, reactances of low value for other frequencies will by analogy have to be connected. Notwithstanding the fact that input and output are in this case periodically entirely disconnected it may therefore be said in this case that at the first-mentioned terminals (output) the voltage is a function of current and voltage at the second-mentioned terminals, whereas at these latter terminals (input) the current is a function of voltage and current at the first-mentioned terminals. These functions then also comprise the instants at which the values are zero.

It should be observed that in the modulators described the transposing frequency does not or only very slightly participates in the energy exchange and that the energy exchange between the frequency (frequencies) to be transposed and the transposed frequency (frequencies) is practically without losses.

Taking the above into account. the field of apnlication of the invention may thus be described by combining the above requirements. In a general manner the invention may then be defined by stating, that in such a frequency transposer the connected elements will have low impedances to undesired frequencies at those terminals where the currents are dependent on the quantities at other terminals, whereas these impedances are high at other terminals.

This description will also comprise all those transposers realizing the invention, but which are distinct from the simple examples described in the above, e. g. due to the transposer being built up of two partial transposers, which of themselves do not satisfy the requirement, or due to the presence of more than two pairs of terminals, in which case matrix relations will in general exist between those pairs, these relations being reducible in the case of two pairs of terminals (not counting those of the transposing frequency) to a reciprocity relation. The latter may subsidiarily be used as a basis for describing the limits of the invention, such, that at any moment the ratio between input and output currents equals the ratio between output and input voltages. If the field of the invention is thus described, its definition, viz. that in the connected 7 circuit elements high impedances occur for undesired frequencies at one side, such undesired frequencies encountering low impedances if occurring at another pair of terminals, should be supplemented by the further requirement that if at a pair of terminals the current should not be at every moment dependent on current and voltage at the other pair of terminals, then the high impedances should occur at that same pair of terminals. This additional requirement has to be made on account of the possible use of interruption modulators, the terminals of which are periodically shorted.

The above property, according to, which a transposer embodying the invention does not occasion reflection points, may be usefully applied in a number of circuits. Such a transposer might, for instance, be incorporated in a filter circuit, which to that end would be opened up at a certain point, such point being arbitrarily subject to the requirement that the nature of the impedances to both sides of the transposer conform to the conditions given above as long as provisions have been made that a frequency transposition will have occurred behind the transposer, so that the filter elements in the filter part concerned will have to be suitably transformed with respect to the elements in the remaining portion of the filter. As an example of such a transposition filter, a band filter will be described below consisting of an initial part and a latter part with a frequency transposer arranged bet\veen,the initial part passing the band from f1 to f1+f2, and the latter part passing the band from f1+fo to f1+f2+fo. As will appear from What follows, special considerations apply to the construction of the latter part in order to obtain suitable adaptation of this part to the initial part for the region of desired frequencies.

Various preferred embodiments of the invention will now be described in connection with the accompanying drawings, in which:

Figure l is a diagrammatic illustration of an inversion modulator comprising contiguous filters;

Figure 2 is an equivalent or explanatory diagram of an inversion modulator, by reference to which certain fundamental relations can be readily explained;

Figure 3 illustrates one embodiment of transposition filter according to the invention;

Figure 4 is an explanatory diagram similar to Figure 2;

Figures 5 and grams;

Figure 7 is an explanatory circuit diagram;

Figure 8 is a curve of certain values in a constant-K filter; and

Figures 9, l0 and 11 diagrammatically illustrate other arrangements of filter parts, transposer, etc.

7 In Fig. 1, S is an inversion modulator of known type made up of contact rectifiers, the polarisation voltages of which are controlled by the transposing frequency in. The inversion modulator is connected to an input filter I and an output filter B. The elements shown connected inter se and with an incoming and an outgoing line by transformers 1-6. As is well known, it is assumed that the voltage of frequency f0 has a considerable amplitude, whereby currents arriving on the winding 3 and having a single harmonic frequency f1 will be converted into currents in winding 4 consisting of a frequency spectrum containing components fi-l-fo, i-fn etc.

The discussion will in the first instance be limited to the case where the winding 1 is connected to a low-frequency incoming line. The filter I may then be a low-pass filter. We further suppose a carrier transmission line to be connected to the winding 6; the filter B may then be a band filter with a pass range from e. g. It to fO+f1. The frequency f1 may be taken as representative of the low frequency alternations (e. g. of a speech band) arriving on the low frequency line. If we take for instance the case 6 are explanatory curve diaof a long carrier frequency line with terminal installations conforming to Fig. 1 (one of these of course inverted), then low frequency lines may immediately be connected tothe low-pass filters, and thus a speech channel might be formed via the carrier line, the said channel only comprising simple filters and inversion modulators; the latter hardly have any losses, so that amplification of the currents is not required.

According to the invention, then, the low damping in the inversion modulators is obtained by making the impedance of the filter I as seen from the winding 2 low for frequencies outside the pass range. and the impedance of the filter B high for frequencies outside the pass range. Of course the alternative might also be the case.

The existence of these relations will now be demonstrated with reference to the equivalent diagram of Fig. 2.

In Fig. 2 the supply frequency and the frequencies formed at the input are shown in the rows and the frequencies produced at the output are shown in the columns. Only a few of the infinite number of frequencies produced are shown.

Originally only the frequency ii is supplied; as will appear immediately the retroaction of output modulation products of f1 and ft on the input will there cause, among others, by retroaction, the frequencies f1+2fo and f12fo, for which reason these latter frequencies are shown at the side of the frequency ii.

The diagram represents the interaction between voltages and currents of different frequency as if their frequencies were equal. The transformers, which are illustrated here in a symbolical sense, have a transforming as well as a transposing action, the latter being however without interest to the interaction as such. Of course, if the interaction with other apparatus is considered the transposition will have to be taken into account, so that for instance the impedances to be connected in the diagram to the terminals f13fo should be calculated to the frequency f1--3fc; if this frequency is negative this should also be taken into account.

The values shown by the side of the transformer windings show the magnitude of M/L and thus indicate the transformer ratio and the sign of the interaction.

In the case in question, it has been assumed that the columns correspond to the output and that here short-circuits exist for a number of undesired frequencies. As will appear further on, the efiiciency will then be a maximum if the input (the rows) has open circuits for undesired frequencies. It is supposed that the windings connected to the output terminals for a certain frequency (in the diagram) are all in parallel, whereas those for the input terminals are all series-connected; by calculating now the equivalent diagram in this manner-which, a priori, is as well justified as inversely paralleling the input windings and series-connecting the 01.1tput windingsan important simplification is obtained in the calculation, due to limitation in the number of unknowns. From such calculation, the diagram may be shown to be correct.

The use of the diagram may, for instance, be exemplified by assuming the above condition to be fulfilled, viz. that with a view to reducing the losses, the output presents shorts to a number of undesired frequencies such as f13fo. J1Jo, f1+3fo, f1+5f0 etc. so that only f1+fo remains. Then the prevention of energy losses at the input in the sense of the invention is obviously only possible by there providing open circuits for the undesired frequencies f1+2fo, f12fo etc., taking care that only the frequency f1 finds an impedance differing from zero.

It is to be observed that it has been necessary in this idealized case to assume these rigorous requirements, but that a real modulator will have an internal resistance which will have to find its place in the diagram of Fig. 2. A considerable improvement in efficiency is already possible when the impedances in question only satisfy considerably less stringent requirements, e. g. to the effect that only the most important undesired frequency will encounter shorts or open circuits.

These suppositio-ns being satisfied, the ratio between the voltages f1 and f1+fo will be determined by the central transformer and thus the voltage at f1+fo will be 1r/2 as great as that at f1.

By way of application of a transposer or converter according to: the invention, Fig. 3 shows a band filter containing a transposer or converter.

The filter consists of an initial part BI and a latter part BU between which a. transposer S is introduced. The part BI is designed for passing a band of frequencies from ii to f1+f2; the part BU for transmitting a band from f1+fo to f1+f2+fc. In the transposing device S the transposition occurs from the first mentioned band to the latter band.

As appears from the figure, the initial part ends in a complete shunt impedance and the latter part begins with a complete series impedance. In this manner the requirement respecting the values of the impedances to undesired frequencies is automatically satisfied. This requirement might, of course, also be satisfied by entirely separating the initial and the latter part and by ending one part at mid-shunt and the other at mid-series. But then the transmission through S would seriously be impaired at the limits ,of the range to be transmitted, because the impedance of the mid-series network approaches zero at the limits, then again to increase outside the pass range. On the contrary, the impedance of the mid-shunt network approaches infinity at the limits, before it decreases. It will be clear that an efficient adaptation at the border frequencies can in no way be obtained. Reverting to the example given in Fig. 2 for the desired frequency the ratio of the absolute impedance values for optimum energy transfer would there have to be equal to 1 1, the impedances having to be conjugated; if a band has to be transferred it thus is desirable that such an optimum relation be maintained as constant as possible over the entire band. These requirements may be satisfied to a high degree by incorporating the modulator in the filter in the manner shown in Fig. 3.

A filter made up like that of Fig. 3 may also be designed for the transition of low frequency signalling to double side band, in which case the impedance ratio should however be 1 /2, as may at once be seen from the equivalent diagram.

In order to show the improvement made by the invention it may be remarked that the total attenuation of a modulator if connected between resistances, is equal to 1n 7r/2+b, I) being the attenuation of the modulator proper; and if the invention is applied it equals Area sinh 1r/2 sinh b. When using an inversion modulator made up of known rectifier cells, the limit at which the old circuit is equivalent to that according to the invention is not attained by a long way, so that an important improvement appears to be attained.

The invention is applicable to power transposers or converters as well as to transposers or converters for communication systems. The invention thus also refers to a transposition filter permitting the formation of combination frequencies and filtering the same; and furthermore refers to a filter therefor, the general nature of which has been described above. In such a filter, both the initial and the latter parts might exercise a filtering action; viz., the initial part may have a low frequency selective action and the latter part a high frequency selective action for separating the desired transposed frequency from the undesired frequencies resulting from the transposition. It has been described above how the frequencies produced at the transposer output will occasion by retroaction on the input the desired (i. e., the originally supplied) frequencies on this side as well as a number of undesired frequencies. It has been described how satisfying certain requirements regarding the impedances for undesired frequencies in the initial and latter parts may permit the attainment of maximum efficiency of the filter; it has been indicated that the impedances of the parts on both sides of the transposer should be mutually adapted, taking into account the transformer action of the transposer for that frequency or those frequencies.

In simple modulation processes as applied in most transposers, the side bands are produced in pairs. If single side band transmission is desired one of those side bands will be a desired frequency (group) and the other an undesired frequency (group). It has been discovered that in the absence of suitable precautions, in some cases the side band situated in the attenuation region of the latter part filter will exert an infiuence on the impedance of this filter to the side band situated in the pass range of the filter, the adaptation of the initial and latter parts being thereby disturbed.

The invention consists in this aspect mainly in means for preventing this undesired effect, the nature of which means may best be explained from the diagram shown in Fig. 2, by retaining only the rows for f1 and the columns for ,71-10 and f1+fo; thus the diagram of Fig. 4 is obtained. The filter connected to the transposed output is represented in the diaphragm by two ideal filters for the side bands; it is seen that these are in this case series-related. This means that their real parts, after having been calculated for the transposed frequencies there present, and after having been added, must, after taking into account the transformer ratio of the transposer, be substantially equal to the output impedance of the initial part as seen from the transposer, this impedance having to be calculated for the transposable frequencies there present. The requirement that the attenuating action of the latter part filter for some frequencies should not exert a disturbing influence on the pass action of the same filter for other frequencies, may be satisfied to a high degree by providing for the compensation in so far as possible of the reactive impedance elements at the input of the latter part by reactive components of the impedance of the initial parts as seen from the transposer, these components having again to be calculated for the frequencies present at the respective points.

The above will be clarified by reference to some examples. Thus Fig. 5 shows the real parts I of the impedance of a latter part as a function of the audio-frequency, as well as the reactive components 8 and 9; the carrier is supposed to lie at the border of the band. The filter in question has, for its admittance, the midshunt admittance of a constant-K filter completed by a shunt-branch of conductivity equal to 0.3 of that of the full shunt-branch of that constant-K filter.

If the side band fi-fo only is passed, the other side band will fall into the attenuation range and the part ode will therefore be absent. If part of this side band is also passed, the part cd of the curve shown may be present. Fig. 6 shows, for a certain case, the components of the initial part as seen from the transposer. According to the invention the sum of the reactive impedance components (taking into account the occurring transpositions and transformations) is as near as possible equal to zero.

Some ways of attaining this desired result follow immediately from the diagram given. It need only be considered that the mutual disturbance in the present case corresponds entirely to that occurring in the case of Fig. 7, if, to the band filters there shown, the pass and attenuation ranges of which are mutually reciprocal, a frequency is applied which is passed by the one filter and attenuated by the other. The characteristic impedance of the source may be a resistance R; as far as the real components are concerned, the impedance of the pass filter need, in this case only, be reckoned with; the real component of this impedance should equal R.

It is known that the real components may be adapted as much as possible by constructing both filters at their input as :r:0.8 filters or by completing them to such terminations. A corresponding artifice applied to the input impedance of the latter part might also result in improvement in the present transposition filter. However the imaginary components should, in the two analogous cases, also be considered. In order to explain this it is desirable to examine the actions at the basis of the artifice just mentioned. In the above description, the symbol .r is used as follows: (1) when the filter terminates on a shunt branch, :1: signifies the ratio of the admittance of this branch to the admittance of the full shunt branch in the filter; (2) when the filter terminates on a series branch, at signifies the ratio of the impedance of this branch to the impedance of the full series branch in the filter.

Fig. 8 shows the real and the imaginary parts of a certain constant-K filter. The real component evidently varies rather strongly over its entire frequency range (the pass range), which fact in general is objectionable if the filter must be combined with other elements. It may be observed that it is in general desirable to construct filters, and in particular also the transposition filter according to this part of the invention, in such a manner that their input impedances at the input and the output show a given characteristic normalized for filters in general. This characteristic may be more or less flat, as is shown by the dot-dash line in Fig. 8 for the case there shown. The dot-dash line may be obtained from the full-drawn curve for the filter in question, of which Fig. 9 shows the input end, by paralleling the first shunt branch by a shunt branch having conductivity equal to 0.3 of that full shunt branch of that constant-K filter. This complementary shunt branch is shown at I; together with the half shunt branch or originally terminating the constant-K filter at mid-shunt it makes up the x=0.8 filter termination. The filter thus terminated has, as was purposed, a more constant real component of the impedance in the pass range; but the branch f causes the impedance in the pass range to be no longer purely real, adding reactive components to the same; on the other hand the reactive components in the attenuation range are changed. For this reason it is usual to add also a series branch when completing a filter to :r=0.8, that series branch having for its object to compensate the reactive components in the pass range. Thus for such a complete filter, conditions for transmission in the pass range are again normalized. But it is clear that if the inputs of a number of such filters having different pass ranges are supplied from a common source the series branches h in the individual filters may be omitted and the said reactive components, which will partially cancel out, may be compensated for the remaining part by an impedance connected to the common input circuit.

In the case of the present transposition filter, this principle may quite easily be applied by constructing the filter for instance with an initial part comprising, as seen from the transposer, an inductance-preferably equal to 1.4/1r of the inductance present in the series branch of the constant-K band filter having the same nominal characteristic impedance and the same band width as the filter used in the latter part-optionally in series with a condenser for establishing resonance for a low frequency in the pass range, this inductance, o. q, this resonant circuit having to be followed by an impedance with the least possible reactance and equal to 4/1 times the characteristic impedance of the band filter. Other forms of the initial part are also possible, for instance when in designing the transposition filter and in starting the design from normal constant-K filters, these are not normalized as in the above example to constant and mutually equal real impedance components, but when equality of these components is deemed sufficient. In this case also supplementary series reactances will be required to compensate for reactive components which have not been cancelled out.

So far I have, in explaining this aspect of the invention, supposed that the ideal filters would be series-related, this supposition amounting to assuming for the other undesired frequencies (besides the other side-band) at the side of the band filter low impedances, and at the side of the low-pass filter high impedances. Inverting these conditions will result in a parallel connection in the equivalent diagram. Then the above considerations remain valid if the dual transformation of impedances into admittances and inversely is performed.

Fig. 10, which shows considerable similarity to Fig. 9, illustrates a transposition filter with input terminals at the left hand side, and at the right hand side the beginning of the band filter constituting the latter part. This beginning thus shows similarity with the parts 1 and g of Fig. 9. The transposer T is shown connected before this beginning, and said transposed is supplied with the frequencies to be transposed via the circuit element 12' and with the transposing frequency S. The circuit element It has the same function as the element h in Fig. 9, which was described above.

It should be observed that other embodiments of the invention are also possible, i. e. by connecting in parallel to a filter having constant-K impedance, a resonant circuit having a resonant frequency between the carrier and the geometric mean of the band filter, the value L/C for this circuit differing from that value for the half shunt-branch of the constant-K filter. The circuit thus obtained will have an impedance, the real part of which strongly resembles the midseries constant-K impedance of a low-pass filter having the same band width as the band filter, this real part being then represented as a function of f1 (in the manner of Fig. 5). For the imaginary components consequently some series elements will again have tobe added in order to compensate them. A filter thus obtained is shown in Fig. 11; where X represents the said series elements, m the half shunt branch of the filter and 7c the added shunt branch.

It should be observed that in the above, the transponator has been considered to be loss-free;

if the losses are taken into account this would occasion small changes in the values.

Having thus described my invention, what I claim as new, is:

1. In a frequency transposing system, the combination of a low-pass filter connected with a signal source, a band-pass filter connected with a load circuit, and transposing means coupled with said filters and supplied with a transposing frequency, said band-pass filter having an input impedance corresponding substantially to that of a constant-K filter characterized by a 0.8 input shuntbranch, said low-pass filter comprising a series inductance adjacent to the transposer having 1.4/1r of the inductance present in the series branch of the constant-K filter having the same nominal characteristic impedance and band width as said band-pass filter.

2. In a frequency transposing system, the combination of a low-pass filter connected with a single source, a band-pass filter connected with a load circuit, and transposing means coupled with said filters and supplied into a transposing frequency, said band-pass filter having an admittance substantially equal to that of a constant-K filter having a 0.8 shuntinput branch, said lowpass filter comprising adjacent to the transposer a parallel capacity of 1.4/1r the capacity present in the shunt-branch of the constant-K filter having the same nominal characteristic impedance and band width as said band-pass filter.

3. In a frequency transposing system, the combination of a low-pass filter connected with a signal source, a band-pass filter connected with a load circuit, an input transformer comprising a primary winding connected with said low-pass filter and a secondary winding provided with an intermediate tap, an output transformer comprising a secondary winding connected with said band-pass filter and a primary winding pro vided with an intermediate tap, a source of transposing frequencies connected with said intermediate taps, and a network of rectifiers establishing unilateral conductivity from the ends of said output transformer primary to the ends of said input transformer secondary, and establishing transposed unilateral conductivity from the ends of said input transformer secondary to the diagonally opposite ends of said output transformer primary, said band-pass filter comprising a series inductance at its input end presenting high impedance to undesired frequencies, said low-pass filter presenting low impedance to undesired frequencies.

4. In a frequency transposing system, the combination of a low-pass filter connected with a signal source, a band-pass filter connected with a load circuit, an input transformer comprising a primary winding connected with said low-pass filter and a secondary winding, an output transformer comprising a secondary winding connected with said band-pass filter and a primary winding, a network of rectifiers establishing unilateral conductivity from the ends of said output transformer primary to the ends of said input transformer secondary, and establishing transposed unilateral conductivity from the ends of said input transformer secondary to the diagonally opposite ends of said output transformer primary, and a source of transposing frequencies connected with said network, said band-pass filter having an input impedance corresponding substantially to that of a constant-K filter characterized by a 0.8 input shunt-branch; said lowpass filter comprising a series inductance having 1.4/1r of the inductance present in the series branch of the constant-K filter adjacent to the transposer having the same nominal characteristic impedance and band-pass width as said band-pass filter.

5. The method of filtering and transposing frequencies wherein one or more input frequencies are transposed to one or more desired output frequencies by the action of a transposing frequency and wherein the input voltages are dependent on the output currents and voltages and at the same time the output currents are dependent upon the input currents and voltages, which comprises supplying the input frequencies through an input filter section, discharging the output frequencies through an output filter section, supplying the transposing frequency to transposing means coupled between said filter sections, and presenting low impedance to undesired frequencies in one of said filter sections adjacent to the transposer and high impedance to undesired frequencies in the other filter section adjacent to the transposer while retaining normal filtering characteristics, whereby the transposlng frequency participates only to a slight extent in the energy transfer, and whereby the energy transfer between the input frequency or frequencies and the output frequency or frequencies is substantially free of losses.

6. Filtering means for use as an input and output of a system comprising a low frequency source and a modulator, said filtering means having an output termination in the form of a bandpass filter including an input impedance corresponding specifically to that of a constant-K filter characterized by a 0.8 input shunt-branch, and a low-pass filter in the input comprising a series inductance adjacent to the modulator having 1.4/1 of the inductance present in the series branch of the constant-K filter having the same nominal characteristic impedance and band width as said band-pass filter.

7. A system of the class described comprising a frequency transposer acting to establish correspondence between different current/voltage phenomena, each phenomenon being determined by a definite frequency and a definite set of transposer terminals, certain phenomena being moreover desired in the system and certain other phenomena being undesired, the phenomena being divisible into two groups A and B, such that the current values of the A phenomena are completely determined, due to the transposer action, by the current and voltage values of the B phenomena, whereas the voltage values of the B phenomena are likewise determined by the current and voltage values of the A phenomena, the algebraical sum of the energy fiuxes into the transposer due to the action of the combined phenomena being practically Zero, networks being connected to the transposer such that in as high a degree as possible for the undesired phenomena, the A phenomena are limited to their current aspect and the B phenomena to their voltage aspect.

8. A system according to claim 7 characterized in that the transposer action is such that the transposer may be considered to be replaced by a set of m columns of mn transformers grouped in 11. rows (if the A group contains m phenomena and the B group n phenomena), each of the transformers having a frequency transposing tion (from one of the m A phenomena to one of the n B phenomena) as well as a voltage and current transforming action, the primary windings of the row transformers corresponding to a single B phenomenon being connected in series, and the secondaries of the n column transformers belonging to a single A phenomenon being connected in parallel.

9. A system according to claim '7 characterized in that for those undesired phenomena to which complete application of the rule of claim 7 has not been possible, as well as for the desired phenomena, the networks joined to the transposer are such that at the junctions for as many of the desired phenomena as possible, and otherwise for suitable combinations of phenomena including desired phenomena, conditions of optimum energy transfer exist, or are approximated, these implying joined impedances being conjugate complexes,

10. In a filter for frequency converting systems of the class described, the combination of a low-pass filter portion in the input and a band-pass filter portion in the output, said bandpass filter portion having an admittance equal to that of a constant-K filter having an 0.8 input shunt-branch, said low-pass filter portion comprising a parallel capacity of 1.41r the capacity present in the shunt branch of a constant-K filter having the same nominal characteristic impedance and band width as said band-pass filter portion.

FREDERIK HENDRIK STIELTJ ES.

CERTIFICATE OF CORRECTION.

December 211., 19LLO.

FREDERIK HENDRIK STIELTJES.

Patent No. 2,226,026.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 6, first column, lines 26 and 27, claim ii, strike out the words "adjacent to the transposer" and insert the same before "having" in line 214., same claim; and that the said Letters Patent should be read with this correction therein that the same may, conform to the record of the case in the Patent Office.

Signed and sealed this Lush day of February, A. D. 19in.

Henry Van Arsdale, (Seal) Acting Commissioner of Patents. 

